Process for the Preparation of an Electrode from a Porous Material, Electrode Thus Obtained and Corresponding Electrochemical System

ABSTRACT

Process for the preparation of electrodes from a porous material making it possible to obtain electrodes that are useful in electrochemical systems and that have at least one of the following properties: a high capacity in mAh/gram, a high capacity in mAh/liter, a good capacity for cycling, a low rate of self discharge, and a good environmental tolerance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/561,845, filed on Jan. 29, 2007, which is a National Stage ofApplication No. PCT/CA2004/000956, filed on Jun. 25, 2004, which claimsthe benefit of Canadian Application No. 2,432,397, filed on Jun. 25,2003. The entire contents of each of U.S. application Ser. No.10/561,845, Application No. PCT/CA2004/000956 and Canadian ApplicationNo. 2,432,397 are hereby incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The purpose of the invention is to provide processes for the preparationof electrodes from a porous material, in particular processes for thepreparation of electrodes involving the preparation of an alloy, andthose processes during which the electrodes are at least partly coatedwith carbon.

It is also a purpose of the invention to provide electrodes which areobtained from a porous material or those which contain a porousmaterial, in particular negative electrodes for lithium micro-batteriesthat contain porous silicon.

Another purpose of the present invention consists in any electrochemicalsystem that contains at least one electrode that is obtained from aporous material, or one that contains a porous material, and moreparticularly electrochemical systems that contain micro-batteries madeof at least one electrode according to the invention.

DESCRIPTION OF THE STATE OF THE ART

Generators based on polymer electrolytes that were recently developeduse metallic lithium, sometimes sodium, or other alkali metals, as anodesheets. Alkali metals are malleable and can be used in the form of thinfilms as mentioned in the Patents CA-A-2,099,526 and CA-A-2,099,524.

However, when using metallic lithium or other alkali metals, there aresome risks that lithium or the alkali metal may melt and that theelectrochemical cell may be destroyed, in certain cases of extreme use,for example at temperatures higher than 100° Celsius. Moreover, underinduced conditions of electrochemical cycling, the formation ofdendrites, for example lithium dendrites, may take place, for example inthe presence of recharge currents that are too high. The formation ofdendrites is associated with many disadvantages, while the same alloydoes not allow for the deposit of lithium or dendrite growth, when itoperates at a more anodic voltage, for example at a voltage whose valueis between +300 and 450 mVolts in the case of lithium aluminum vslithium.

The use of alkali metal alloys, for example lithium, is thus describedin the U.S. Pat. No. 4,489,143, in the case of generators that operatein molten salt media.

In organic media, and more particularly in a polymer medium, where thethicknesses of the electrode films are less than 100 micrometers, itbecomes very difficult to use alloyed anode sheets (also called alloybased sheets). Indeed, the intermetallic compounds of lithium that canbe used as anodes, such as LiAl, Li₂₁Si₅, Li₂₁Sn₅, Li₂₂Pb₅ and the like,are hard and brittle and cannot be laminated as it is the case withlithium or low alloyed lithium. On the other hand, it is mentioned inCanadian Patent 1,222,543, that these anodes can be prepared in the formof thin films by producing composites consisting of powders of theintermetallic compound bound by the polymer electrolyte, or still, inU.S. Pat. No. 4,590,840, that it is possible under certain conditions,to pre-lithiate the sheet of the host metal of the anode, by chemicallytreating the surface thereof or by electrochemically charging part ofthe sheet.

However, although these techniques are practical under certainconditions, they rely on reactive materials, and the pre-inserted alloysare often pyrophoric. In addition, they are not easily implemented andit is difficult to optimize their performances. When the anodes areprepared in discharged state, one of the major difficulties to overcomeoriginates from the important voluminal variation which results from theformation of the alloy, which causes important stresses on thestructure.

When an attempt is made to produce the alloy from a sheet ofnon-lithiated host metal, during the assembly or after the assembly of apolymer electrolyte generator, voluminal expansion of the structure inthe direction of the thickness of the sheets could only be compensatedby an appropriate design of the cell, for example by adapting the totalincrease of the thickness of the superimposed sheets, the more sobecause in the direction of the thickness, the variation is very smalland is therefore much more negligible.

Known techniques for the preparation of micro-batteries in anelectrochemical system are described in the conference, 1th ABA BRNO2000, Advanced Batteries and Accumulators, June 28.8-1.9, 2000 BrnoUniversity of technology, Antoniska 1, Brmno, Czech Republichttp://www.aba-brno.cz/aba2000/part_I/13-bludska.pdf, and in thereference: Bull. Master. Science., Vol. 26, No. 7, December 2003, pages673-681 http://www.ias.ac.in/mastersci/bmsdec2003/673.pdf, and requirethe introduction of the micro-battery into the system.

A need therefore existed for new materials that can be used asconstitutive element of an electrode and that are free of one or more ofthe disadvantages of the materials traditionally used in thisapplication.

In particular, there was a need for a new electrode material having atleast one of the following properties:

-   -   a high capacity in mAh/gram;    -   a high capacity in mAh/litre;    -   a good capacity of cycling;    -   a low rate of self-discharge; and    -   a good environmental tolerance.

There was also a need for new electrode materials that are adapted foruse in micro-technology, such as in micro-batteries.

Moreover, there was a need for electrodes in which there are very fewcracks after manufacture and this, for purposes of longevity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-4 shows the mechanism for inserting lithium into silicon througha process according to the invention, with formation of a Li_(x)Si_(y)alloy.

FIG. 2-4 shows the mechanism for the electrochemical formation,according to an embodiment of the invention, of porous silicon that isused as an anode for micro-batteries.

FIG. 3-4 relates to the manufacture of a micro-battery according to theinvention starting with a porous silicon based electrode that is coatedwith carbon. The role of this carbon is to establish an electrochemicalbridge between silicon and lithium. Moreover, carbon makes sure that theporous silicon based negative electrode is electronically conductive. Itis also used to provide adhesion together of the particles of siliconduring voluminal expansion.

The various steps of this manufacture are the following:

1—deposits of thin layers of photo-resist;

2—placing a mask, and passing a UV beam to cross-link the desired zones;

3—dissolving non cross-linked zones by chemical stripping;

4—carbonizing the non stripped photo-resist (that will form the anode inthe micro-battery);

5—introducing the electrolyte, followed by the cathode;

6—cutting operation in stripped zones to provide micro-batteries.

FIG. 4-4 relates to an optical picture of a carbon structure obtained bylaser pyrolysis of a layer of photo-resist. The top part of the graphrepresents a Raman spectrum of the carbon obtained.

SUMMARY OF THE INVENTION

Processes for preparing electrodes for electrochemical system from aporous material that allows the preparation of electrodes forelectrochemical systems showing very interesting properties with respectto their physico-chemical as well as mechanical performances. Thetechnology that is presented allows for the production ofmicro-batteries, directly in electrochemical circuits.

GENERAL DESCRIPTION OF THE INVENTION

A first object of the present invention resides is a process for thepreparation of an electrode for an electrochemical system from a porousmaterial. Preferably, the porous material used has a porosity, measuredby the hereinafter referred to mercury method, that varies from 1 to99%, terminals included. More advantageously still, the porosity of thematerial varies from 20 to 80%, terminals included.

According to an advantageous embodiment of the invention, the porousmaterial used is such that the average pores found therein vary from 1nanometer to 1 micrometer, terminals included. More preferably still,the size of the pores varies from 10 to 250 nanometers, terminalsincluded.

According to another advantageous embodiment, distribution of the poresis substantially uniform. Preferably, this distribution is selected sothat its d50 is between 100 and 150 nanometers.

The pores are advantageously present at the surface of the porousmaterial and extend throughout said porous material. Preferably, thepores have a depth between 1 micrometer and 3 millimeters and saidporous material has a thickness between 2 micrometers and 3.5millimeters. It is desirable that most of the pores that are found inthe porous material do not extend throughout the entire porous material.

According to another preferred embodiment, said porous material isselected among materials capable of providing an alloy with an alkalimetal. Thus, the porous material may be selected from the groupconsisting of silicon, tin, aluminum, silver, gold, platinum andmixtures of at least two of these materials, when they are in porouscondition.

Preparation of the alloy is carried out by chemical and/orelectrochemical means.

It was surprisingly realized that particularly interesting results areobtained when the void ratio of the material used to prepare theelectrode is such that the voids of the porous material can absorb thevoluminal expansion generated during the production of the alloy withthe alkali metal.

According to a particular embodiment of the invention, an anode isprepared from porous silicon.

An anode according to the invention may thus be obtained by preparationof an alloy from at least one source of porous silicon and at least onealkali metal selected from the group consisting of Li, Na, Ca andmixtures of at least two of these metals.

Advantageously, an anode is prepared from porous silicon, in which theporosity measured according to the mercury porosimeter method, variesfrom 5 to 95 volume %, terminals included. More advantageously still,the porosity of the silicon used is about 75 volume %.

The porous silicon used as porous material is obtained from a source ofsilicon selected from the group consisting of: silicon wafers, siliconpellets, silicon films and mixtures of at least two thereof.

Preferably, the porous silicon used as porous material is obtained froma silicon monocrystal.

According to an advantageous embodiment of the invention, the poroussilicon is obtained from a source of silicon, by electrochemicaltreatment, in a bath comprising at least one salt, said salt preferablybeing selected from the group consisting of NH_(x)F_(y) wherein X is 4or 5 and Y is 1 or 2, more preferably still the selected salt is NH₄F.

By way of example, the treatment of the source of silicon contains atleast one salt in solution, that is preferably a mixture of H₂SO₄, NH₄Fand H₂O, and at least one non aqueous solvent that is preferably analcohol or a ketone, the non aqueous solvent(s) is (are) preferablyselected from the group consisting of methanol, ethanol and acetone, andmixtures of at least two of these solvents.

Such a bath advantageously contains, in volume:

-   -   10 to 60% NH₄F;    -   5 to 20% methanol; and    -   75 to 20% H₂O.

Preferably, the alloy is based on porous silicon and is in the form ofSi_(x)Li_(y), wherein x represents a number between 1 and 5 and yrepresents a number between 5 and 21. More preferably still, in thealloy, x represents about 4 and y represents about 21.

According to an advantageous embodiment of the invention, the alloyproduced is of the Si_(x)Li_(y) type, and it is obtainedelectrochemically by contacting a source of silicon with lithium and/ormetallic lithium in the form of sheets or wafers, at a temperaturebetween 40 and 100° Celsius, preferably at a temperature of about 80°Celsius.

The time of contact of the source of silicon with lithium and/ormetallic lithium in the form of sheets of wafers, is comprised between 1and 12 hours, preferably, said time of contact is about 3 hours.

A second object of the present invention consists of the electrodesobtained by implementing a process according to any one of the processesdescribed in the first object of the invention.

An advantageous sub-family of anodes according to the invention consistsof the anodes containing at least 60 weight percent and preferably 40weight percent of a porous material, preferably porous silicon.

Another particularly interesting sub-family consists of the anodes,which are at least partly coated with carbon.

The anodes and cathodes according to the invention are advantageous inthat they are substantially free of cracks.

A third object of the present invention consists in electrochemicalsystems such as those that include at least one electrode as obtained byany one of the processes of production defined in the first object ofthe invention or as defined in the second object of the invention.

By way of preferred example of particularly preferred electrochemicalsystems, batteries in which the electrolyte is of the liquid, gel, orpolymer type, may be mentioned.

In these batteries, the cathode is preferably of the type LiCoO₂,LiFePO₄, LiNiO₂, LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.33)Mn_(0.33)O₂, and thecathode is preferably of the 1 to 5 Volts type.

Batteries of the rechargeable type, providing particularly significantperformances, may thus be obtained. Preferably they are of the lithiumion type.

A sub-family of batteries of the invention that are of interest consistsof those in the form of micro-batteries, preferably those whosedimensions are between 1 mm² and 10 cm², and which possess at least oneof the following electrochemical properties:

-   -   electrochemical capacitance higher than 1 μWh;    -   cycling capacity higher than 500, preferably higher than 100        cycles;    -   self-discharge rate lower than 5%, preferably lower than 4%,        more preferably still lower than 3%; and    -   a life span, measured by the storage test carried out under        ambient conditions, that is higher than 3 years, preferably        higher than 5 years.

A fourth object of the present invention relates to the use of theelectrodes, and preferably the anodes of the invention, in anelectrochemical system.

Preferably, the anode is used as negative electrode for lithiummicro-batteries.

A fifth object of the present invention relates to processes for themanufacture of an electrode, that is based on porous silicon and is atleast partly coated with carbon, as obtained by thermal pyrolysis of apolymer layer, that is preferably deposited as a thin layer on apreferably insulating support made of porous silicon such as Si₃N₄.Pyrolysis of the polymer is advantageously carried out at a temperaturebetween 600 and 1100° C. and, preferably, for a period of time between30 minutes and 3 hours.

According to another variant of the invention, when implementing theprocess of manufacture of an electrode based on porous silicon and whichis at least partly coated with carbon, there is provided a laserpyrolysis of a polymer layer preferably coated as a thin layer on asilicon (insulating) support. The beam used preferably has an intensityof between 10 and 100 milliwatts and it is preferably placed at adistance of between 0.5 micrometers and 1 millimeter from the siliconsupport. The layer of photo-resist is carbonized by laser pyrolysis byexposing the layer to the latter, which means that the C—H—O functionsare converted into carbon. Preferably, the exposition is carried out fora period of time between 1 second and one minute. Preferably, thesilicon support consists of a silicon monocrystal and it has a thicknessbetween 100 microns and 3 millimeters.

A sixth object of the present invention consists of the electrodesobtained by implementation of one of the processes defined in the fourthobject of the invention.

A seventh object of the present invention consists of theelectrochemical systems including at least one electrode according tothe fifth object of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention concerns the use of a porous material in amicro-battery. More particularly, the invention relates to anelectrochemical generator including a negative electrode comprising aporous host metal, such as silicon. The host sheet metal being intendedto later on constitute a negative electrode and having the property ofabsorbing the lateral expansion and of substantially preventing anychange in the plane of the porous metal during formation of an alloybetween the host metal and the alkali metal.

For example, after formation of the lithium host metal alloy, the alloycracks when there is electrochemical activity. The possibility of avoluminal expansion plays a preponderant role for the integrity of theelectrode.

Porous silicon is thus advantageously used in this technology, as theactive material that constitutes the anode for a Li-ion battery. Thetheoretical capacitance of porous silicon is 1970 mAh/grams and 2280mAh/l.

The voluminal expansion associated with the alloy of silicon and lithiumis preferably between 30 and 40%. Thus, the voids formed in poroussilicon are used for compensating the voluminal expansion of the Li andSi based alloy.

The mechanism for chemical or electrochemical insertion of lithium intoporous silicon, according to an embodiment of the invention, isillustrated in FIG. 1-4.

The empty space generated by the porosity of silicon is occupied by thevoluminal expansion of the Si_(x)Li_(y) alloy, wherein x varies from 1to 5 and y varies from 4 to 21. Preferably, the alloy has the formulaLi₂Si₅.

Preparation of Porous Silicon

Within the framework of the processes for preparing porous siliconaccording to the invention, a mixture of NH₄F is advantageously used todissolve Si and SiO₂ that are present as impurities.

Porous silicon is obtained by electrochemical means in an electrolytebased on NH₄F (50%)+H₂O+Methanol in a ratio of (2:2:1), the addition ofmethanol allows the formation of hydrogen at the surface. The void ratiois calculated according to the rate of blending of lithium that isproportional to the voluminal expansion of the Si_(x)Li_(y) alloy.Porosity is measured by the mercury method described in the reference:The Powder Porosity Characterisation La bat NYS

College of Ceramics at Alfred University, Jun. 18, 2002,http://nyscc.alfred.edu/external/ppc/ppc.html.

This technique is described more in detail in the field ofsemi-conductorshttp://etd.caltech.edu/etd/available/etd-08062002-192958/unrestricted/Chapter3.pdf,in chapter 3 of the Ph.D thesis entitled Effects OF SURFACE MODIFICATIONON CHARGECARRIER DYNAMICS AT SEMICONDUCTEUR INTERFACES by Agnes Juang,2003, California Institute of Technologie Pasadena, Calif.

On the other hand, it was surprisingly found that micro-batteries,according to the invention, using micro-electrodes containing carbon andbased on porous silicon, can be manufactured by different originaltechniques explained hereinafter in detail.

The technique in question is thermal pyrolysis, or one obtained by lasermeans, of the heteroatoms of the type N or O that are present at thesurface of silica, and that were deposited as thin layers on theinsulating material also called “the insulator”.

The two techniques imply the conversion of the C—H—O functions intocarbon, however they differ in procedure with respect to the formationof microstructures that constitute the microelectrodes in themicro-battery. In the two techniques, the starting material ispreferably commercial silicon called dense silicon that is available inthe form of wafers.

According to an advantageous approach, the techniques for thepreparation of semi-conductors imply the “patterning” of carbon byphotolithographic methods involving a photo mask that is used to “model”the electrode structures, that can be interdigitized electrode surfaces.

First Technique for Preparing an Electrode Based on Porous Silicon

A conventional “wafer” transfer silicon with an insulating layer may beused as substrate for the microelectrodes. According to this innovativeapproach, carbon electrodes are formed from regular photo-resists byheat treatment (normally at temperatures of 600 to 1100° Celsius in aninert atmosphere during one hour) that carbonizes them and makes thephoto-resist electrically conductive.

The electrochemically active electrode materials may be selectivelydeposited on the carbons by electrochemical methods, and for certainapplications, carbon itself may be used as electrode.

The process that is used to manufacture micro-electrode structures relyon a series of steps. In the first step, a fine layer of Si₃N₄ (about100 nm) is deposited by chemical vapor deposit (CVD), which is used asinsulator to separate the conductive silicon “wafer” from the carbonatedstructure. Subsequent steps involving spin coating, “patterning” andphoto-resist pyrolysis, are used to form the final carbonated structure.The negative as well as the positive photo-resist are used to form thecarbonated conductive micro-electrodes.

These techniques are described in the references: 253, IMLB 12 Meeting,2004 The Electrochemical Society, Journal of Power Sources, Volume: 89,Issue: 1, Jul., 2000 and Applied Physics Letters Vol 84(18) pp.3456-3458. May 3, 2004.

Second Technique for Preparing an Electrode Based on Porous Silicon

The second approach does not involve the use of a photo mask. Indeed,only a narrow laser beam with “path” is used, and the latter iscontrolled to move according to a specific “path” trajectory. Control ofthe movement of the laser beam on the surface of the photo-resist bycomputer control authorizes the preparation of a wide variety ofmicro-electrode devices comprising channels. The power intensity of thelaser vapor is controlled so as to prevent vaporization of thephoto-resist, instead of its conversion into carbon, and this alsominimizes the loss of carbon by laser ablation. In a subsequent step,the photo-resists that have not reacted in certain portions that are notexposed to laser vapor, are dissolved to leave only the carbonatedmicro-electrodes on the silicon wafer. Radiation from the laser beam canconvert the photo-resist polymer into carbon. Similarly, results thatare comparable to those obtained by thermal pyrolysis, are obtained byusing a Raman spectrum.

The document How Semiconductors are made E. Reichmanis and O. Nalamasu,Bell Labs, Lucent Technologies, Intersil, Jun. 20, 2003, IntersilCorporation Headquarters and Elantec Product Group, 675 Trade Zone Blvd,Milpitas, Calif. 95035 describes silicon wafers (and their preparation)that can be used within the framework of the present invention.

The document of Martin Key, entitled SU-8 Photosensitive Epoxy, CNM,Campus UAB, Bellaterra 08193, Barcelona, Spain(http://www.cnm.es/projects/microdets/index.html) illustrates methodsallowing to deposit a carbonated polymer on silicon wafers.

The document Direct Measurement of the Reaction Front in ChemicallyAmplified Photoresists, E. Reichmanis and O. Nalamasu, Bell Labs, LucentTechnologies, Sciences, 297, 349 (2002) describes currently used methodsfor attacking selected zones of photo-resists.

The content of these three documents is incorporated by reference intothe present application.

EXAMPLES Carbon Micro-Structures Obtained by Laser Pyrolysis ofPhoto-Resist are Described Hereinafter

A photo-resist (Oir 897-101, Olin Corp., Norwalk, Conn.) was used toproduce a thin film of an organic precursor on a Si substrate. A LabramRaman microscope integrated system manufactured by the group ISA Horibawas used for a laser pyrolysis treatment of the photo-resist, and alsoto analyze the structure of the carbonated product.

The excitation wave length was either supplied by internal HeNe (632nm), a laser mW or by an external Ar-ion (514 nm), 2/Laser. The power ofthe laser beam was adjusted to the desired levels with neutral filtersof diverse variable optical densities.

The size of the laser beam at the surface of the sample can be modulatedfrom 1.6 up to a few hundreds of microns; and it is controlled by thecharacteristics of the optical microscopes and the distance between thesample and the lenses. The diameter of the laser beam that is used forour experiments was 5 microns. To control the position of the samplewith respect to the laser beam, a XY microscope with motorized scanningand a resolution of 0.1 micron, was used. The time of exposure of thephoto-resist to the laser beam was controlled either by the scanningspeed XY, or by a shutter digital laser beam (model 845 HP by NewportCorp.), which was used in static experiments.

Control of the movement of the sample of photo-resist—Si by the computerprogram makes it possible to design a wide variety of micro-electrodesin the form of rows.

The power density of the laser beam should be regulated in order toprevent vaporization of the photo-resist without converting it intocarbon or to minimize carbon loss by laser ablation.

Four layers of a positive photo-resist were coated on a Si wafer, afterwhich they were baked at 150 degrees Celsius. An optical picture thatillustrates the result obtained by laser pyrolysis for the production ofa carbon structure from a positive photo-resist, is shown in the annexedFIG. 4-4. A laser operating at 632 nm, and with a beam size of 5micrometers and a capacity of 8 mW, was used to produce the modeledcarbons. A computer program was used to control the movement of thesample and to form the carbon configurations. The movement speed of themotorized sample XY was 8 mm per second. The width of the print in theinterdigitalized structure was about 20 micrometers. It is slightlywider than the carbon bond that connects them together since each fingerwas exposed twice to the laser beam. The Raman spectrum of the carbonpositioned on one of the fingers is shown on the same FIG. 4-4. It isremarkably similar to the one obtained by standard thermal pyrolysis ofthe same photo-resist at 1000 Celsius. These preliminary results showthat micro-electrodes of carbon having a Raman spectrum that compares tothose obtained by heat treatment, are easily obtained.

A new highly performing technology which is based on the use of laser isthus proposed for the preparation of capacitances whose size is adaptedto electronic devices of small size.

This new method of the invention called—direct laser lithography(DDL)—permits the production of micro-electrodes from organic andinorganic precursors, that are adapted for Li-ion batteries and theproduction, from any type of substrate, of functional micro-batteriesthat are completely rechargeable.

Versatility of the DLL technology allows for the production on requestof micro-power sources that can be distributed and directly integratedto electronic components.

Moreover, the DDL technology requires no photo masking to give thedesired configuration to the micro-electrodes.

Consequently, DLL may produce a design of micro-battery more rapidlythan conventional photolithography. The micro-batteries thus producedprovide improved specific energy and capacity by reason of their weightand their reduced volumes, when the electronic substrate becomes part ofthe elements of the battery.

The cathode may be prepared from a target of cathode material preferablyselected from the group consisting of LiCoO₂, LiMn₂O₄,LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂, LiMn_(1/2)Ni_(1/2)O₂, LiMPO₄ (M=Fe, Co,Ni, Mn) and mixtures of at least two of these materials, preferably thetarget material is pressed, the laser is applied on the target atcapacities that can vary from 20 mW to 2 W to produce the porousmaterial that constitutes the cathode that is thereafter stripped fromthe target with a laser and deposited on the porousSi/carbon/electrolyte half battery.

A second technique for preparing the cathode using a laser, is carriedout from a compound in paste form prepared from a mixture of a cathodepowder and a carrier solution that is preferably toluene, heptane or amixture thereof. The pasty solution is coated on a support plate that ispreferably made of glass and placed 100 μm from the substrate (siliconor others). The UV radiation laser beam is applied through the supportplate and the cathode is projected on the substrate by pyrolysis.

Although the present invention has been described by means of specificembodiments, it is understood that many variations and modifications canbe associated with said embodiments, and the present invention aims atcovering such modifications, uses or adaptations of the presentinvention following in general, the principles of the invention andincluding any variation of the present description that will becomeknown or is conventional in the field of activity in which the presentinvention resides, and which may apply to the essential elementsmentioned above, in accordance with the scope of the following claims.

1. Process for the preparation of an electrode for an electrochemicalsystem from a porous material wherein the porous material is poroussilicon.
 2. (canceled)
 3. Process for the preparation of an electrodeaccording to claim 1 in which the porosity of said material varies from20 to 80%, terminals included.
 4. Process for the preparation of anelectrode according to claim 1 in which the average size of the pores insaid porous material varies from 1 nanometer to 1 micrometer, terminalsincluded.
 5. Process for the preparation of an electrode according toclaim 4 in which the size of the pores varies from 10 to 250 nanometers,terminals included.
 6. Process for the preparation of an electrodeaccording to claim 1 in which the distribution of the pores issubstantially uniform, and such that its d50 is between 100 and 150 nm.7. Process for the preparation of an electrode according to claim 1 inwhich the pores are located at the surface of the porous material andextend throughout said porous material; the pores have a depth between 1μm and 3 mm and said porous material has a thickness between 2 μm and3.5 mm.
 8. Process for the preparation of an electrode according toclaim 1 in which said pores do not extend entirely throughout the porousmaterial.
 9. Process for the preparation of an electrode according toclaim 1 in which said porous silicon is capable of forming an alloy withan alkali metal.
 10. (canceled)
 11. Process for the preparation of anelectrode, according to claim 9 in which the preparation of the alloy iscarried out by chemical and/or electrochemical means.
 12. (canceled) 13.(canceled)
 14. Process for the preparation of an electrode according toclaim 9 in which the electrode is an anode and said anode is obtained byformation of an alloy from at least one source of porous silicon and atleast one alkali metal selected from the group consisting of Li, Na, Caand mixtures of at least two of these metals.
 15. Process for thepreparation of an electrode, according to claim 1 in which the electrodeis an anode and the anode is based on porous silicon, wherein theporosity, measured according to the porosimeter mercury method, variesfrom 5 to 95 volume %, terminals included.
 16. Process for thepreparation of an anode, according to claim 15 in which the porosity isabout 75 volume %.
 17. Process for the preparation of an anode,according to claim 14 in which the source of porous silicon is selectedfrom the group consisting of: silicon wafers, silicon pellets, siliconfilms and mixtures of at least 2 thereof.
 18. Process for thepreparation of an anode according to claim 14 in which the poroussilicon is obtained from a silicon monocrystal.
 19. Process for thepreparation of an electrode, according to claim 1 in which the electrodeis an anode and the porous silicon is obtained from a source of silicon,by electrochemical treatment, in a bath comprising at least one saltselected from the group consisting of NH_(X)F_(Y) wherein X is 4 or 5and Y is 1 or 2, more preferably still the selected salt is NH₄F. 20.Process for the preparation of an anode according to claim 19 in whichthe bath contains at least one salt in a solution that is a mixture ofH₂SO₄, NH₄F and H₂O, and of at least one non aqueous solvent that isselected from the group consisting of methanol, ethanol, acetone andmixtures of at least 2 of these solvents.
 21. Process for thepreparation of an anode according to claim 20 in which the bathcontains, in volume, from: 10 to 60% NH₄F; 5 to 20% methanol; and 75 to20% H₂SO₄.
 22. Process for the preparation of an anode according toclaim 14 in which the porous silicon based alloy is in the form ofSi_(x)Li_(y), wherein x represents a number between 1 and 5, and yrepresents a number between 5 and
 21. 23. Process for the preparation ofan anode according to claim 22 in which x represents about 4 and yrepresents about
 21. 24. Process for the preparation of an anodeaccording to claim 22 in which the alloy formed is of the Si_(x)Li_(y)type, and it is obtained by electrochemical means by contacting a sourceof silicon with lithium and/or metallic lithium in the form of sheets orwafers, at a temperature between 40 and 100° C.
 25. Process according toclaim 24 for the preparation of an anode, in which the time of contactbetween the source of silicon and metallic lithium is between 1 and 12hours.
 26. Anode obtained by implementing a process according toclaim
 1. 27. Anode characterized in that it contains at least 40% byweight of a porous silicon.
 28. Anode according to claim 27, at leastpartly coated with carbon.
 29. Electrochemical system including at leastone anode as defined in claim 27, at least one cathode and at least oneelectrolyte.
 30. Electrochemical system in the form of a batteryaccording to claim 29, in which the electrolyte is of the liquid, gel,or polymer type.
 31. Electrochemical system according to claim 30 thatis a battery in which the cathode material is LiCoO₂, LiFePO₄, LiNiO₂,LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.33)Mn_(0.33)O₂, and the cathode is of the1 to 5 Volts type.
 32. A rechargeable battery which is anelectrochemical cell according to claim
 31. 33. Battery according toclaim 32 in the form of microbattery, having dimensions between 1 mm²and 10 cm², and which has at least one of the following electrochemicalproperties: electrochemical performances: an electrochemical capacityhigher than 1 μWh; a capacity of cycling higher than 500 cycles; aself-discharge rate lower than 5%, and a life span, according to thestorage test carried out under ambient conditions, higher than 3 years.34. Electrochemical system including at least one anode as defined inclaim 26, at least one cathode and at least one electrolyte
 35. Arechargeable battery according to claim 33, which is a lithiummicro-battery
 36. Process according to claim 1 for manufacturing aporous silicon based electrode, which comprises a further step ofthermal pyrolysis of a layer of polymer that is coated in thin layer ona thin insulating support of porous silicon such as Si₃N₄, pyrolysis ofthe polymer being carried out at a temperature between 600 and 1100° C.and for a period of time between 30 minutes and 3 hours.
 37. Processaccording to claim 1 for manufacturing a porous silicon based electrode, which comprises a step of laser pyrolysis of a layer of polymer coatedin thin layer on a silicon (insulating) support, the laser beam havingan intensity between 10 and 100 mW and being placed at a distance ofbetween 0.5 μm and 1 mm from the silicon support, during a period oftime between 1 second and one minute.
 38. Process according to claim 36in which the silicon support consists of a silicon monocrystal and ithas a thickness of between 100 microns and 3 millimeters.
 39. Electrodeobtained by implementing a process according to claim
 36. 40.Electrochemical system including at least one electrode according toclaim
 39. 41. Process for the preparation of an anode, according toclaim 1 in which the porous silicon is obtained from a source ofsilicon, by electrochemical treatment, in a bath comprising at least onesalt, wherein said salt is NH₄F.
 42. Process for the preparation of ananode according to claim 19 in which the alloy formed is of theSi_(x)Li_(y) type, and it is obtained by electrochemical means bycontacting a source of silicon with lithium and/or metallic lithium inthe form of sheets or wafers, at a temperature of about 80° C. 43.Process for the preparation of an anode according to claim 24 in whichthe time of contact between the source of silicon and metallic lithiumis about 3 hours.
 44. The anode of claim 26, wherein it contains atleast 40% by weight of porous silicon.
 45. The anode of claim 26,wherein it contains at least 60% by weight of porous silicon. 46.Battery according to claim 32 of the rechargeable lithium ion type.